Conquest is a far easier mode to start out in.You have the whole map opened up.So not squashed into a smaller killing zone like on Rush.
Do not capture a flag alone until you get confident on what is happening.
Use cover,it is all around you.Look around before you move from your cover.The flag capture zones are big,stay on the edge looking in from a nice cover spot.

Jumping into a BF game so late is not going to be an easy pill to swallow.
Everyone knows the maps,weapons and vehicles like the back of their hands.

Though this might be a good time to start, as there seems to be a lot of new Xmas players just marching out into the danger zone.

Great advise!
Also try not to get frustrated, which will not be easy joining bf3 this late. Just try and enjoy and you will get better. Once it clicks, it will most likely be one of the most unforgettable gaming experiences you will ever have.
Get that mic on, find a good squad and enjoy!